While microwave radars have in the past been utilized for obtaining target track information, radars operating in the microwave region of the electromagnetic spectrum suffer from what is commonly known as ground clutter when objects, such as missiles or planes approach the radar at close to ground level. Ordinarily, a microwave radar cannot detect objects closer than approximately 1/2 an antennalobe above the horizontal. This usually means that those objects traveling within 3 to 5 degrees of the horizon cannot be detected by present day microwave radar systems.
As proposed herein, it is suggested that microwave radar coverage be enhanced by the utilization of a "pulse-burst" optical radar which can detect targets within milliradians of the horizon by virtue of better diffraction performance of reasonable sized antennas. The use of the "pulse burst", which is a group of closely spaced pulses, permits the most recent information to be utilized in determining the target track, instead of a time averaged track produced by mono-pulse optical radars. Thus, as will be seen, the "pulse burst" system gives the radar a "vernier" performance characteristic which permits real time target track updating. This is especially important when the target is executing evasive maneuvers to break radar lock. Thus, the use of the "pulse burst" optical radar permits the optical radar to acquire and obtain the track of an incoming target with sufficient accuracy for fire control purposes or any other tracking purposes. It may be utilized either as an adjunct to conventional microwave radars or, for certain situations, completely in place of the microwave radar.
In general, it is also possible with optical radars to achieve finer angular resolution than is possible with microwave radars. In the subject system the angular resolution of the optical radar is on the order of 5 arc seconds. With a 20.times.20 element receiving array and currently available miniature detectors, as will be described hereinafter, the angular resolution of the optical radar may be brought down to 25 microradians which is an order of magnitude improvement over the accuracy of present day optical radars.
Fine angular and range resolution of the subject system is achieved by the utilization of a pulse burst of two or more pulses in which the target is illuminated successively by each of these pulses or "strobed". Thus, the target may be strobed with three pulses and the movement of the target between the first and second, and second and third pulses affords what is called "vernier range resolution", where the term "vernier" relates to the fineness of the range resolution which is obtainable. Vernier range is detected by the utilization of oppositely directed delay lines in which the signal representing the return from the target propagates in one direction down one delay line, whereas a signal representing the initial pulse, after suitable course range delay, propagates in the opposite direction down the other delay line in the pair. Cross correlation at various points along the delay lines indicates at what point in time there is coincidence between the two counter-propagating pulses. As will be appreciated, the time of correlation defines the vernier range in the boresight direction.
Strobing also permits vernier angular resolution. In order to obtain vernier angle information, a segmented receiver is utilized in which each segment is defined by the field of view of a detector. By virtue of the strobing, the output of each detector on which returned radiation impinges is correlated with a particular sampled pulse which permits the vernier range to be correlated with angular displacement from boresight. In order to correlate vernier range with angular displacement, a pair of oppositely directed delay lines is dedicated to each segment of the receiver, with the segments determining the angle off boresight.
Thus, depending on the vernier range correlation position and which of the pairs of oppositely directed delay lines carries a return signal from a target, a waterfall display may be driven and the track of the incoming target can be presented in terms of angle off boresight versus vernier range. The waterfall display, by virtue of its timing, determines the track of the target. This same timing can be introduced into a computer which senses the time of the correlation to compute such a track, and to produce control signals, such as r, r and .theta., .theta. for a coordinate system suitable for describing the position and track of the target.
In one embodiment, the segmented receiver is provided with zoom optics in the telescope to vary the field of view and thus the angular resolution of each detector element. In one operative embodiment the field of view of each detector element can be varied from 25.degree. to 2.degree., thereby to provide the optical radar with variable resolution.
In order to avoid any ambiguity due to correlations of a returned pulse with the wrong sample pulse as the pulses counter-propagate, the lengths of the delays of the delay lines are set to be less than the delay between the pulses in the burst, e.g. the interpulse spacing. It will be appreciated that as the interpulse spacing increases, the radial (r) and angular (.theta.) velocity resolution decreases. However, assuming an interpulse spacing of 10 milliseconds, assuming a mach 1 target at 1500 feet, then the vernier range is good to 10 feet and the angular resolution=.+-.0.38.degree.. This resolution requires shift register delay lines operating at a frequency of 10.sup.7 or higher.
Delay lines for resolving shorter interpulse spacings include tapped coaxial delay lines and acoustic delay lines.
Conventional shift register delay lines can be used if color-coded pulses are used and multiple receivers are provided. In this embodiment the individual receivers respond only to one color. This approach is made feasible through the use of multi-color, multi-pulse lasers, in which the lasing medium lases at a number of wavelengths, and in which a separate Q-switched cavity is provided for each color. Control of the Q-switches in each cavity controls the interpulse spacing. Thus, each pulse is color coded and distinguishable at the receiver site. Distinguishability of returned pulses eliminates overlap problems and permits the use of currently available shift register type delay lines operating at 10.sup.5 -10.sup.6 Hertz.
Without color coding and with conventional shift register delay lines in a 3 pulse burst type system, 3 parallel trajectories will be displayed or detected. If targets do not cut across several angle channels at once, with proper calibration and "zeroing" of the system, one of the trajectories may be selected. When the target does cut across several angle channels at once, as would be the case with a very rapidly moving target, color coding (or other similar coding such as polarization coding) is effective to sort out the time sequence of pulse returns.
In an alternative system, which uses pulse bursts and oppositely directed delay lines, radial velocity in a given angular channel can be obtained utilizing the cross correlation approach by taking adjacent transmitted pulses and running them down oppositely directed delay lines to obtain interpulse spacing dT, and by taking adjacent returned pulses and running them down oppositely directed delay lines to obtain the received interpulse spacing .DELTA.T. dT and .DELTA.T are obtained by detecting at which pairs of taps cross correlated (integrated) outputs appear. Radial velocity is then easily evaluated as ##EQU1##
It is therefore an object of this invention to provide an enhanced accuracy optical radar;
It is a further object of this invention to provide an optical tracking system which is better able to direct a tracking platform to keep a selected fast-moving target on boresight;
It is a still further object of this invention to provide an optical radar capable of following evasive maneuvers of a target;
It is another object of this invention to provide vernier control for an optical radar system;
It is another object of this invention to provide an enhanced accuracy optical radar with vernier control and/or variable resolution;
It is a still further object of this invention to provide an optical radar utilizing a sectored receiver and oppositely directed delay lines for enhanced accuracy and rapid determinations of radial and angular velocity;
It is yet another object of this invention to provide an enhanced accuracy optical radar in which the target is pulse strobed to permit vernier range resolution;
It is another object of this invention to provide pulse bursts and oppositely directed delay lines for rapid radial velocity measurements.
It is a yet still further object of this invention to provide enhanced accuracy optical radar utilizing multiple color-coded pulses and oppositely directed delay lines coupled to a sectored receiver.
These and other objects of the invention will be better understood in connection with the following description in view of the appended drawings in which: